Noise reduction is often challenging, in machine systems generally and in turbomachinery in particular. Acoustic noise-reduction devices must be designed to ensure sufficient and effective noise reduction in a given frequency range of interest, but also comply with integration constraints. These include temperature, weight, drainage, structural behavior, installation, damage prevention, and so on.
However, of the solutions that have been developed over the years, few are well adapted to broadband noise reduction.
To this end, so-called micro-channeled materials have been developed. A micro-channeled material is a structural material formed of thin-walled-metal tubes disposed in an array, such as a honeycomb-like structure.
U.S. Pat. No. 7,963,364 describes an example of such a structure, wherein the micro-channel structure comprises an array of tubes each having a nominal diameter of between approximately 100 μm and 300 μm.
Such a structure is advantageous, in that it allows for good noise attenuation over a wide frequency range, in particular for frequencies above 1 kHz. Moreover, the thin walls of the micro-channels offer a significant weight reduction with respect to insulation structures based on more conventional narrow-band Helmholtz-type resonators.
These micro-channel structures are fabricated by plating nickel metal onto a polymer wire mandrel in a thin layer, thereby forming the micro-channels. The coated wire is then placed in a crucible. The crucible is heated under a strong vacuum, to approximately 400° C., at which point the polymer material of the wire mandrel breaks down and is ingested by the vacuum pumping system. After leveling off for approximately one hour, the crucible is then heated to approximately 1200° C. The temperature is leveled off for long enough to allow the micro-channels to fuse to each other, and then cooled.
However, using a strong vacuum is disadvantageous: in order to protect the vacuum pumps, which are easily contaminated by the products of the pyrolysis, elaborate filtration systems must be developed and maintained. Furthermore, furnace design and material selection are also difficult, not only in terms of maintaining the vacuum seals but also due to the fact that since carbon is favorably deposited on high-temperature surfaces during decomposition, any exposed heating elements will experience significant fouling which ultimately results in electric arcing and failure.
It is therefore an object of the disclosure herein to resolve at least some of the above-mentioned issues.